A research programme on the mechanical behaviour of a homogenous volcanic tuff found in the centre of the city of Naples (Italy) was carried out at the University of Naples a few years ago. Isotropic and drained triaxial tests were performed in a very wide range of confining pressures (up to 60 MPa). After presenting the stress-strain curve pattern and the mean stress influence on the shear behaviour, the paper focuses on the definition of a strength criterion and of the yield surface for this material. Some tuff samples were subjected to isotropic compression tests up to a confining pressure approximately twice as high as the isotropic yield stress; they were subsequently unloaded and subjected to drained triaxial tests. Partial loosening of the interparticle bonds (“destructuration”) was observed. The paper also compares the mechanical behaviour of intact and “destructured” samples, emphasising the effects of the structure on strength and yield.